Add glow::Context to epi::Frame (#1425)

This can be used, for instance, to:

* Render things to offscreen buffers.
* Read the pixel buffer from the previous frame (glow::Context::read_pixels).
* Render things behind the egui windows.

egui_web/src/backend.rs

@@ -155,6 +155,7 @@ impl AppRunner {
             },
             output: Default::default(),
             storage: Some(Box::new(LocalStorage::default())),
+            gl: painter.gl().clone(),
         };
 
         let needs_repaint: std::sync::Arc<NeedRepaint> = Default::default();
@@ -274,12 +275,14 @@ impl AppRunner {
         Ok((needs_repaint, clipped_primitives))
     }
 
+    pub fn clear_color_buffer(&self) {
+        self.painter.clear(self.app.clear_color());
+    }
+
     /// Paint the results of the last call to [`Self::logic`].
     pub fn paint(&mut self, clipped_primitives: &[egui::ClippedPrimitive]) -> Result<(), JsValue> {
         let textures_delta = std::mem::take(&mut self.textures_delta);
 
-        self.painter.clear(self.app.clear_color());
-
         self.painter.paint_and_update_textures(
             clipped_primitives,
             self.egui_ctx.pixels_per_point(),

egui_web/src/glow_wrapping.rs

@@ -32,6 +32,10 @@ impl WrappedGlowPainter {
 }
 
 impl WrappedGlowPainter {
+    pub fn gl(&self) -> &std::rc::Rc<glow::Context> {
+        self.painter.gl()
+    }
+
     pub fn max_texture_side(&self) -> usize {
         self.painter.max_texture_side()
     }
@@ -48,7 +52,7 @@ impl WrappedGlowPainter {
         self.painter.free_texture(tex_id);
     }
 
-    pub fn clear(&mut self, clear_color: Rgba) {
+    pub fn clear(&self, clear_color: Rgba) {
         let canvas_dimension = [self.canvas.width(), self.canvas.height()];
         egui_glow::painter::clear(self.painter.gl(), canvas_dimension, clear_color);
     }

egui_web/src/lib.rs

@@ -339,6 +339,7 @@ fn paint_and_schedule(runner_ref: &AppRunnerRef, panicked: Arc<AtomicBool>) -> R
     fn paint_if_needed(runner_ref: &AppRunnerRef) -> Result<(), JsValue> {
         let mut runner_lock = runner_ref.lock();
         if runner_lock.needs_repaint.fetch_and_clear() {
+            runner_lock.clear_color_buffer();
             let (needs_repaint, clipped_primitives) = runner_lock.logic()?;
             runner_lock.paint(&clipped_primitives)?;
             if needs_repaint {